The default value

intrinsic_matrix contains valid initial values of fx, fy, cx, cy that are optimized further. Otherwise, (cx, cy) is initially set to the image center (image_size is used here), and focal distances are computed in some least-squares fashion

The optimization procedure consider only one of fx and fy as independent variable and keeps the aspect ratio fx/fy the same as it was set initially in intrinsic_matrix. In this case the actual initial values of (fx, fy) are either taken from the matrix (when CV_CALIB_USE_INTRINSIC_GUESS is set) or estimated somehow (in the latter case fx, fy may be set to arbitrary values, only their ratio is used)

The principal point is not changed during the global optimization, it stays at the center and at the other location specified (when CV_CALIB_FIX_FOCAL_LENGTH - Both fx and fy are fixed. CV_CALIB_USE_INTRINSIC_GUESS is set as well)

Tangential distortion coefficients are set to zeros and do not change during the optimization

The focal length is fixed

The 0-th distortion coefficient (k1) is fixed (to 0 or to the initial passed value if CV_CALIB_USE_INTRINSIC_GUESS is passed)

The 1-st distortion coefficient (k2) is fixed (see above)

The 4-th distortion coefficient (k3) is fixed (see above)